Show conference changelog

Use load_and_authorize_resource in versions controlller
Add conference specifc route to revision history page
Users with role can view revision_history only for the versions they have access to
Handle versions where conference_id is not set (records before papertrail was introduced)

app/views/admin/versions/index.html.haml

@@ -1,6 +1,16 @@
 .row
   .col-md-12
     .page-header
+
+      .dropdown.pull-right
+        %button.btn.btn-success.dropdown-toggle{ 'data-toggle' => 'dropdown', type: 'button' }
+          = @conference.nil? ? 'All Conferences' : @conference.short_title
+          %span.caret
+        %ul.dropdown-menu
+          %li= link_to 'All Conferences & Users',  admin_revision_history_path
+          - @conf_ids_with_role.each do |conference_short_title|
+            %li= link_to conference_short_title,  admin_conference_revision_history_path(conference_id: conference_short_title)
+
       %h1 Revision History
       %p.text-muted
         Log of changes made to conferences and associated resources